I just noticed that microsoft has also released the source to MS-DOS 2.0: https://github.com/microsoft/MS-DOS
I don't think I ever imagined that I'd be able to "look behind the curtain" like this, let alone that Microsoft would come to embrace open source.
From there I moved on to Turbo Pascal 3.0, and then Borland Pascal 7.0 (which I loved - super fast compiler and superb IDE - many examples and comprehensive documentation - which were a must in the pre-Internet era), then Borland C. And then Linux and gcc for the following two decades.